1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a plurality of fuel cell units that are formed by sandwiching an electrode assembly between separators.
2. Description of the Related Art
Among fuel cell units forming fuel cell stacks, there is one type that is formed in a plate shape by sandwiching between a pair of separators a membrane electrode assembly that is formed by placing an anode electrode and a cathode electrode respectively on either side of a solid polymer electrolyte membrane. A fuel cell is formed by stacking in the thickness direction of the fuel cell units a plurality of fuel cell units that are constructed in this way.
In each fuel cell unit there are provided a flow passage for fuel gas (for example, hydrogen) on one surface of the anode side separator that is positioned facing the anode electrode, and a flow passage for oxidizing gas (for example, air that contains oxygen) on one surface of the cathode side separator that is positioned facing the cathode electrode. In addition, a flow passage for a cooling medium (for example, pure water) is provided between adjacent separators of adjacent fuel cell units.
When fuel gas is supplied to the electrode reaction surface of the anode electrode, hydrogen is ionized there and moves to the cathode electrode via the solid polymer electrolyte membrane. Electrons generated during this process are extracted to an external circuit and used as direct current electrical energy. Because oxidizing gas is supplied to the cathode electrode, hydrogen ions, electrons, and oxygen react to generate water. Because heat is generated when water is created at the electrode reaction surface, the electrode reaction surface is cooled by a cooling medium made to flow between the separators.
The fuel gas, oxidizing gas (generically known as reaction gas), and the cooling medium each must flow through a separate flow passage. Therefore, sealing technology that keeps each flow passage sealed in a fluidtight or airtight condition is essential.
Examples of portions that must be sealed are: the peripheries of penetrating supply ports formed in order to supply and distribute reaction gas and cooling medium to each fuel cell unit of the fuel cell; the peripheries of discharge ports that collect and discharge the reaction gas and the cooling medium that are discharged from each fuel cell unit; the outer peripheries of the membrane electrode assemblies; and the outer peripheries between the separators of adjacent fuel cell units. A material that is soft yet also has appropriate resiliency such as organic rubber is employed for the sealing member.
In recent years, however, size and weight reduction, as well as a reduction in the cost of fuel cells, have become the main barriers in progress towards the more widespread application of fuel cells through their being mounted in practical vehicles.
Methods that have been considered for reducing the size of fuel cells include making each fuel cell unit forming the fuel cell thinner, more specifically, reducing the size of the space between separators while maintaining a maximum size for the reaction gas flow passage formed inside each fuel cell unit; and also making the separators thinner.
However, there is a limit to how thin the separators can be made due to the strength requirements for each separator and by the rigidity requirements for the fuel cell. Reducing the height of the sealing members is effective in reducing the size of the spacing between separators; however, the height of the sealing members must be sufficient for the sealing members to be able to be pressed down enough to ensure that the required sealing performance is obtained. Therefore, there is also a limit to how much the height of the sealing members can be reduced.
Furthermore, in a fuel cell unit, although the space occupied by the sealing members is indispensable in order for the reaction gas and cooling medium to be sealed in, because this space contributes substantially nothing to power generation, it must be made as small as possible.
FIG. 24 is a plan view showing a conventional fuel cell stack. In FIG. 24 the reference numeral 70 indicates a communication port such as a fuel gas supply port and discharge port, an oxidizing gas supply port and discharge port, and a cooling medium supply port and discharge port that each penetrate the fuel cell stack in the direction in which separators 71 are stacked. The reference numeral 72 indicates an area in which a plurality of fuel gas flow passages, oxidizing gas flow passages, and cooling medium flow passages running along the separators 71 are formed.
FIG. 25 is a longitudinal cross-sectional view of a conventional fuel cell stack 73 taken along the line Xxe2x80x94X in FIG. 24. As can be seen in plan view, in order to make the space occupied by the sealing member, that does not contribute to power generation, as small as possible, conventionally, by locating gas sealing members 76 and 77, which respectively seal a fuel gas flow passage 74 and an oxidizing gas flow passage 75, together with a cooling surface sealing member 78, which seals a cooling medium flow passage, aligned in a row in the stacking direction of the fuel cell units 79, the outer dimensions in the stacking direction of the fuel cell stack 73 are restrained to the minimum.
However, the drawback with the fuel cell stack 73 that is constructed in this manner is that if the gas sealing members 76 and 77 that seal the flow passages 74 and 75 as well as the cooling surface sealing member 78 are all placed in a row in the stacking direction of the fuel cell unit 79, then the thickness of the fuel cell stack 73 cannot be made less than a value obtained by adding the height of the cooling surface sealing member 78 to the thickness of each fuel cell unit 79, and multiplying this result by the number of fuel cell units stacked in the fuel cell stack.
In order to explain this more specifically, the discussion will return to FIG. 25. According to FIG. 25, the fuel gas supply port 70 and the fuel gas flow passage 74 that are isolated in a sealed state by the gas sealing members 76 and 77 are connected by a communication path 80. The communication path 80 is provided in the separator 81 in the vicinity of the fuel gas supply port 70 so as to detour around, in the thickness direction of the separator 81, the gas sealing member 77 that seals the entire periphery of the fuel gas flow passage 74. Moreover, the separator 82 also has a similar communication path (not shown) in the vicinity of the oxidizing gas supply port (not shown).
Accordingly, each of the separators 81 and 82 are formed relatively thickly in order to form the communication path 80; however, as is seen in the cross section in FIG. 25, at the position of the seal line where each of the sealing members 76 to 78 are placed, the separators 81 and 82 are formed with the minimum thickness needed to ensure the required strength, and it is not possible to make them any thinner.
Moreover, because each of the sealing members 76 to 78 is formed with the minimum height needed to secure the sealing performance, it is not possible to reduce the height of the sealing members 76 to 78 any further.
As a result, although the thickness of the fuel cell stack 73 is found by multiplying the number of stacks by the sum of the minimum thickness of the two separators 81 and 82, the thickness needed to form the communication path 80, the height of the two gas sealing members 76 and 77, the thickness of the solid polymer electrolyte membrane 83, and the height of the cooling surface sealing member 78, because these are all indispensable, it is extremely difficult to achieve any further reduction in thickness.
As a countermeasure for reducing the overall thickness of such a fuel cell stack 73, it is proposed that the gas sealing members 76 and 77 and the cooling surface sealing member 78 be disposed so as to be offset with respect to each other as viewed in the stacking direction. Accordingly, it is possible to greatly reduce the dimension in the stacking direction of the fuel cell stack 73 by reducing the height of the cooling surface sealing member 78 that is needed to ensure the sealing performance, while, on the other hand, the thickness of the portions of the separators 81 and 82 where the communication paths are formed is ensured.
However, by disposing the gas sealing members 76 and 77 and the cooling surface sealing member 78 so as to be offset with respect to each other, the sealing portions of each fuel cell unit are not aligned in a row in the stacking direction. As a result, sealing pressures applied to the gas sealing members 76 and 77 and the cooling surface sealing member 78 that are disposed so as to be offset with respect to each other are reduced. Consequently, when the stacked fuel cell units are tightened in the stacking direction, the reaction force produced by the cooling surface sealing member that is being compressed may deform the separators, which may degrade the sealing performance of the sealing members, and may lead to the leakage of the reaction gases and the cooling medium across the portions of the gas sealing members 76 and 77 and the cooling surface sealing member 78 around the deformed portions of the separators. Because the separators 81 and 82 must be sufficiently thick to ensure the rigidity thereof, it is not possible to make the fuel cell stack 73 any smaller.
The present invention was conceived in view of the above circumstances, and it is an object thereof to provide a fuel cell that has been made lighter and smaller by reducing the thickness thereof, while reliably sealing the respective flow passages using the respective sealing members between the separators and the membrane electrode assemblies that form the fuel cell.
In order to solve the above problems, a first aspect of the present invention provides a fuel cell comprising fuel cell units, the fuel cell units being stacked and having at least one cooling medium flow passage therebetween, and the cooling medium flow passage sealed by a cooling surface sealing member, each fuel cell unit comprising: an electrode assembly formed by disposing an electrode on each side of an electrolyte; separators that sandwich the electrode assembly in the thickness direction thereof; and gas sealing members that are disposed at an outer peripheral portion of the electrode assembly, and that seal respective reaction gas flow passages that are formed between each separator and the electrode assembly and are bounded by the separators and electrode assembly, wherein in each of the separators there are provided reaction gas communication ports and cooling medium communication ports that penetrate each of the separators in the thickness direction thereof, and communication paths that detour around the gas sealing members in the thickness direction of the separators and connect the reaction gas communication ports with the reaction gas flow passages; and the portions of the separators, at which the gas sealing members and the cooling surface sealing member are disposed so as to be offset with respect to each other as viewed in the stacking direction, are supported by support members.
According to the fuel cell of the present invention, because the rigidities of the portions of the separators, at which the gas sealing members and the cooling surface sealing member are disposed so as to be offset with respect to each other, are ensured, it is possible to apply sufficient sealing pressure to the gas sealing members and the cooling surface sealing member that are disposed so as to be offset with respect to each other.
A preferable material for the support members depends on the positions thereof, and an electrical insulation material such as rubber or resin is preferably used for the support members that are disposed between the separators between which the electrode assembly is disposed. On the other hand, when the support members are disposed on the cooling surface of the separators, a conductive material having corrosion resistance such as stainless steel or carbon, or an electrical insulation material such as rubber or resin may preferably be used for the support members. When the support members are disposed between the electrode assembly and the separators, one of the above conductive materials having corrosion resistance is also preferably used. A fuel cell to which the present invention will be applied may be of a solid polymer type, a solid electrolyte type, an alkaline type, a phosphoric acid type, or a molten carbonate type.
In the fuel cell of the present invention, the communication paths in one fuel cell unit and the corresponding communication paths in the adjacent fuel cell unit in the stacking direction may be disposed so as to be offset with respect to each other as viewed in the stacking direction, and preferably, at least a portion of each of the support members may support a portion of the separators at which the communication paths are formed.
Accordingly, because the rigidities of the portions of the separators, at which the communication paths are disposed so as to be offset with respect to each other, are increased, the thicknesses of the portions of the separators may be reduced, and the portions of the separators may be disposed so as to be offset with respect to each other as viewed in the stacking direction.
In the fuel cell of the present invention, each of the support members disposed over the reaction gas flow passages or the cooling medium flow passage may be provided with communication recesses that allow the reaction gases or the cooling medium to flow through.
Accordingly, the reaction gases or the cooling medium can be supplied through the communication recesses, and the rigidities of the portions of the separators, at which the support members are provided, can be increased.